The present invention relates to a motion detection system which detects a moving object on a screen between adjacent frames in an image signal, like a television signal.
In a prior motion detection system for a television signal, an analog image signal is converted to a digital form through sampling and quantizing process, and each image signal is indicated by two dimensional sequences of a numerical information which is called a pixel (picture element).
FIG. 1 shows frames for explanation of detection of picture movement. In the figure, the symbol F.sub.0 denotes the present field, F.sub.2 denotes the second preceding field. As is known, interlace scanning is carried out so that a pair of fields provide a frame. In a motion detection system, a preceding frame which is a second preceding field is used.
It is denoted in FIG. 1 that W(0,0) is the present input pixel, W(-1,0) is the left adjacent pixel which is the preceding sampled point, W(0,-1) is the pixel on the preceding scanning line and on the same sampling position as that of W(0,0). In the field F.sub.2, the pixel X(0,0) locates at the same position as that of W(0,0). The adjacent left pixel of X(0,0) is X(-1,0), the upper pixel is X(0,-1), the right pixel is X(1,0), and the lower pixel is X(0,1). FIG. 1 shows that pixels are arranged on cross points of perpendicular coordinates, but other location like quincuncial location of pixels is of course possible.
First, m (sampling points) .times. n (scanning lines) number of pixels compose a block (m and n are integers). The detection of movement of an object in a picture is performed for a block, but not for a pixel. When a block is too small, the motion detection might have an error because of noise on a picture signal. When a block is too large, a fine movement of an object can not be detected. Further, when a block size is small, the number of blocks increases, the information quantity for transmitting movement of each block increases. Considering the above matters, the reasonable block size is 8 (sampling points) .times. 8 (scanning lines).
When the input block #(0,0)' in the present field is provided, a block which has #(0,0) at the center, and (2k+1) cells in the longitudinal direction, and (2L+1) cells in the vertical direction, are defined as shown in FIG. 2.
A block difference B is then calculated to provide similarity measure between the input block in the present field and the block in the preceding picture. When the difference B is small, the similarity is high, and when the difference B is large, the similarity is low. The example of the equation for providing the block difference B is shown in the equation (1), which provides the square sum of the difference between the pixels in the blocks. ##EQU1## In that equation, B(#(0,0)', #(0,0)) is the block difference between a block #(0,0)' and a block #(0,0). The difference between a block #(0,0)' and another block is also calculated by replacing the other block for #(0,0).
Another equation for providing said difference B is shown in the equation (2), which provides the absolute sum of the difference between two blocks. ##EQU2## The equation (2) is used in the explanation below.
The block differences B(#(0,0)',#(-k,-L)) through B(#(0,0)',#(k,L)) are calculated for all the blocks #(-k,-L) through #(k,L). Then, the value B is tested, and the block which has the minimum difference B is determined as the block which has the largest similarity. When more than two blocks have the same value of difference, the block which has the smaller block number is selected.
For instance, when EQU B#(0,0)',#(2,0)) &lt;B(#(0,0)',#(i,j)) (3)
(where i=-k,k; j=-L,L)
is satisfied, the #(2,0) block is determined to be the most similar to the block #(0,0)', and it is determined that the block #(2,0) moves to the block #(0,0)' in a frame duration, that is to say, it is considered that the former block moves in the left direction by two pixels length in a frame duration.
The above prior art can detect the movement of a block, only when the intensity distribution of a block differs considerably from that of adjacent area, and both the fields have the related blocks.
However, said prior art has the disadvantage of having an error due to the following reasons.
(a) When the intensity change on a screen is small, the movement detection is affected by noise in an input signal.
(b) Even when the intensity change is large, the difference of intensity distribution between the related portions may largely depend upon the location of sampling points and the movement of an object.
(c) No block may be defined at the boundary between a moving object and a background.
An error in detection of a moving block causes deterioration of the picture quality when a picture is interpolated in a television standard conversion system using a motion detection system. An error also arises the problems in coding, and/or processing of motion pictures when a moving object is derived from an original picture.